Centrifugal film evaporating apparatus and method



Nov. 12, 1963 H. w. MAYHEW ETAL 3,110,646

CENTRIFUGAL FILM EVAPORATING APPARATUS AND METHOD 3 Sheets-Sheet 1 FiledAug. 4, 1959 FIG .5

n GMM heo y T 0 m M w E N." m H ATTORNEYS Nov. 12, 19,63 H. w. MAYHEWEI'AL 3,110,545

CENTRIFUGAL FILM EVAPORATING APPARATUS AND METHOD Hurry w, Moyhew'NVENTORS ATTORNEYS F G. 2 g .Temple'ron Nov. 12, 1963 w, MAYHEw r3,110,646

CENTRIFUGAL FILM EVAPORATING APPARATUS AND METHOD Filed Aug. 4, 1959 3Sheets-Sheet 3 Harry W. Moyh ew Hugh E.Templeton INVENTORY BY 7 44 n-q4-... 0c -b ATTORNEYS United States Patent 3,110,646 CENTRIFUGAL FILMEVAPORATING APPARATUS AND METHOD Harry W. Mayhew and Hugh E. Templeton,C nc nnat Ohio, assignors to Vulcan-Cincinnati, Inc., Cincinnati,

Ohio, a corporation of Ohio Filed Aug. 4, 1959, Ser. No. 831,547 5Claims. (Cl. 1596) This invention relates to an apparatus which isparticularly adapted for use in the evaporation of heat-sensitivematerials although it has use in other applications wherein a high rateof heat transfer to a liquid is desired. This invention also relates toan improved evaporation method wherein a high rate of heat transfer to aliquid and intimate contact between a vapor and a liquid are obtained.

There is a constant need in the art for new apparatus which provide formore efficient evaporation. For example, it is known to evaporate a ureamelt in order to reduce its water content so that the melt can then beprocessed to form urea prills which are of low water content,mechanically strong, and of low tendency to absorb water. Such prillsare a highly desirable fertilizer ingredient provided they are of lowbiuret content. Biuret is formed by the decomposition of urea, and thispoints to the need for dehydration of the urea melt under conditionssuch that the decomposition of urea to form biuret is minimized.

It has heretofore been proposed to conduct an evaporation operation in agenerally cylindrical vertically positioned chamber in which the liquidto be subjected to evaporation is fed to a centrally located rotarydevice which throws the liquid against the internal, vertical wall ofthe chamber. Evaporation requires heat and hence the chamber has beenprovided with a heating jacket. Also, means for subjecting the chamberto vacuum have been associated with the chamber, since it is Well knownthat decomposition is reduced by evaporation at low pressures and hencelow temperatures.

In evaporation, the extent of evaporation accomplished in a single stagewill not exceed the extent of evaporation which would be accomplishedunder equilibrium conditions. Hence, Where the extent of evaporation tobe accomplished is large, as in evaporating a urea-water mixturecontaining 25% by weight of water to a urea-water mixture containingonly 0.5% by weight of water, evaporation in stages is indicated. Thismeans that the chamber must be provided with a plurality of rotarydevices and thus means must be provided for transferring the liquidundergoing evaporation from one stage to the next stage.

In the distillation art, it is old to distill in apparatus which isessentially a generally cylindrical, vertically positioned vesselprovided with a centrally positioned, rotary, vertical shaft havingattached thereto a plurality of plates, cones or the like which serve tothrow liquid undergoing distillation against the inner, vertical wall ofthe vessel. Cones are attached to the inside of the vessel and eachserves to collect liquid thrown against the wall and feed the liquid tothe next lower rotary plate, cone or the like.

Design conditions in the evaporation of heat-sensitive materials and indistillation are not strictly analogous. Thus, in evaporation, heat isgenerally supplied to the material undergoing evaporation by means of anexternal heating jacket, whereas in distillation such heat as isnecessary is generally supplied from a reboiler located at the base ofthe distillation column. In evaporating heatsensitive materials, a highrate of heat-transfer must be provided from the external heating jacketto the liquid in order to minimize the adverse eifect of prolongedsubjection of the liquid to elevated temperature.

One might suppose that an eflicient evaporator would be obtained simplyby joining an external heating jacket to the typeof distillationapparatus described above, a high rate of heat transfer from the jacketto the liquid being subjected to evaporation being produced by theturbulence resulting from the impact and circumferential motion of theliquid as it isthrown against the inner wall of the vessel. However, anevaporator is not of good design merely because it provides for a highrate of heat transfer. Thus, in addition, the liquid undergoingevaporation must remain in the evaporator a minimum amount of time, orotherwise undue decomposition of the liquid will take place. And wherespinning devices are employed to throw liquid against the inner wall ofthe vessel, this method of operation, although aiding heat transfer, isinconsistent with minimum residence time. The reason for this is thatthe liquid thrown out by the rotary plates, cones or the like has acircumferential component of motion. As a result, the liquid drainingfrom the inner wall of the vessel, after having been impacted thereon,has a whirling motion, so that the liquid does not drain quickly to therotary plate, cone or the like below. This means that the liquidundergoing evaporation does not drain as promptly as it might to therotary plate or the like below, and hence the residence time of theliquid in the vessel is not as short as it might be.

In accordance with this invention, a heat exchanger design is providedwhereby the deficiency just mentioned can be avoided. Thus, inaccordance with this invention, battling means are associated with themeans whereby liquid, after having been thrown against the heat-exchangesurface by the rotary plate, cone or the like, is drained to the nextlower rotary plate, cone or the like. The bafiling means are of suchdesign as to substantially reduce the rotary motion of the liquid, sothat the liquid will drain promptly to the rotary plate, cone or thelike positioned below. By providing the baffling means, efficient heattransfer and short residence time of the liquid undergoing evaporationare accomplished at the same time. Also, improved liquid mixing isobtained on each rotary device. Hence, in its one aspect, this inventionis an apparatus which is suitable for use as an evaporator and which isof generally cylindrical construction. The vessel is provided with anexternal heat exchanger and also with a centrally located, verticallypositioned rotary shift to which there is attached a plurality of rotarydevices positioned one above the other adapted to project or throwliquid undergoing evaporation against the inner, vertical wall of thevessel. The vessel is also provided with means for collecting the liquidprojected or thrown from each rotary device and feeding it to the rotarydevice directly below it. Associated with such collecting means arebaffling means which arrest the rotary fiow of the liquid imparted to itby the rotary devices after the liquid has been in heat-exchangerelationship with the external heat exchanger, so that the liquid willflow more promptly to the rotary device directly below the collectingmeans.

The rotary devices attached to the rotary shaft are preferably in theform of flat plates having edges which are serrated in such manner thatdifierent points on the edges are positioned at different pointslongitudinally with respect to the shaft. By constructing the rotarydevices in such manner, the liquid is projected from the rotary devicesmore in the form of a spray and this makes for improved contact betweenthe liquid thrown from the rotary devices and the vapor evaporated.

For a more complete understanding of this invention, reference is madeto the accompanying drawings which illustrate an embodiment thereof and.in which:

FIGURE 1 is an elevation showing the general arrangement of theapparatus;

FIGURE 2 is a plm section taken along line 22 of FIGURE 1;

FIGURE 3 is a fragmentary vertical cross-section showing the rotatingshaft, plates attached thereto, collecting means in the form of conesand baffles associated therewith, the section being taken along the line3-3 of FIGURE 2;

FIGURE 4 is a fragmentary plan view of a plate;

FIGURE 5 is a developed elevation showing the edge of a plate;

FIGURE 6 is a section taken substantially along line 6-6 of FIGURE 1;and

FIGURE 7 is a broken-away cross-section taken along line 77 of FIGURE 6.

Referring particularly to FIGURE 1, the apparatus shown is one which isadapted for use in the evaporation of a conventional urea meltcontaining approximately 25% by weight of water to provide asubstantially dehydrated urea containing approximately 0.5 percent byweight of water. As shown, the apparatus is a generally cylindrical,vertically positioned vessel 1, having an outer wall and an inner wall2, the space 3 between the outer wall and the inner wall 2 being anexternal heat-exchange jacket in which steam can be injected throughline 4 and from which condensate can be removed through line 5. Theapparatus is also provided with rotating shaft 6, driven from the bottomby a motor (not shown), a plurality of plates 7 being rigidly attachedto the shaft 6. Urea undergoing evaporation is fed in liquid form to theapparatus through line 8 and is removed by means of line 9 in liquidform. Outlet 10 is provided for the removal of water vapor from theapparatus and is connected to a source of vacuum (not shown). Thenumeral 11 designates a chamber at the base of the apparatus for thedrainage of liquid, dehydrated urea therefrom. Liquid urea Ithrownagainst the inner wall 2 is collected by cones 12 and transferred to theplate below a given cone.

Considering now FIGURE 2 and FIGURE 3, each collecting cone 12 isprovided with a plurality of bafiles 13. Hence, in the operation of theapparatus, liquid is thrown from plate 7 against Wall 2. The impact andwhirling motion of the liquid makes for excellent heat-transfer betweenthe liquid and the heat exchanger 3. After impacting against wall 2, theliquid descends into the cone section 12. The whirling motion of theliquid tends to prevent the descent of the liquid in the cone and, inaccordance with this invention, this difiiculty is obviated by providingbaffles 13. These baflles arrest the whirling motion of the liquid urea,so that it can flow promptly down the cones to the plate below. In thismanner, the residence time of the liquid in the apparatus is reduced andhence decomposition of the urea is minimized.

FIGURE 4 and FIGURE 5 illustrate a particularly advantageousconstruction for the rotary plates 7. Thus, as shown, particularly inFIGURE 5, the edge of the plate is serrated so that various points onthe edge of the plate are at different heights. The effect of formingthe plate in this manner is that the liquid leaves the plate in the formof a band of droplets, the band having a definite depth in thelongitudinal direction of the shaft 6. Hence, more intimate contactbetween liquid descending the apparatus and vapor ascending theapparatus is provided for, this being important where the apparatus isto be used as a mass-transfer as well as a heat-transfer device.

Finally, FIGURE 6 and FIGURE 7 give further details as to the methodwhereby the liquid undergoing treatment is introduced into theapparatus. Thus, line 8 is split into lines 14 and Y15 in order that theflow of liquid to the top plate will be distributed more uniformly.

A typical apparatus of the type shown in the drawings has nine plateswhich are spaced approximately fifteen inches apart on the rotatingshaft and which have a diameter of approximately twenty-three inches.The shaft is rotated at approximately 400 rpm. Such an apparatus cansuitably be used in the evaporation of approximately 240 tons of ureaper day. The pressure in the apparatus is maintained at approximately14.7 p.s.i.a. and steam at a temperature of 366 F. is supplied to theheatexchanger through line 4.

Various modifications can be made in the apparatus which is shown in thedrawings in order to provide further embodiments which fall within thescope of the present invention. For example, the plates need not beserrated as shown, but instead can be perfectly fiat, if desired, orcones can be used in place of plates. Also, a plurality of plates orother distributing means can be attached to the shaft between each coneor other collecting device. Moreover, a flat plate provided withsuitable bames can be used in place of the cones shown in the drawings[for the purpose of transferring the liquid from the heat-exchanger wallto the rotating plate Or the like. The diameter of the rotating platesor the like is preferably slightly smaller than the holes through thecones, since when the apparatus is constructed in this manner it is moreeasily assembled. The shaft can be driven either from the top or fromthe bottom, it can be provided with external or internal bearings and itcan have a fixed or variable speed. Depending upon the use to which theapparatus is to be put, means can be provided for the introduction intothe bottom of the apparatus of a sweep gas, such as flue gas ornitrogen. The apparatus can be designed to operate under vacuum, atatmospheric pressure or at super atmospheric pressure. The apparatus canalso be designed to use conventional 'heating means other than steam,for example, aryl silicates or Dowtherm. On the other hand, a fluidcoolant rather than a heating fluid can be supplied to the heat-exchangejacket, among the fluid coolants being cooling water, refrigerated wateror brine, or evaporating Freons.

The apparatus of the present invention can be used for a variety ofpurposes other than the evaporation of urea melts. For example, theapparatus is useful in the evaporation of solvents from high-boilingmaterials and plasticizers which are heat-sensitive. Also, the apparatuscan be used in the processing of other heat-sensitive materials such astall oil, fatty acids and resin acids. In addition, the apparatus findsapplication in the depitching of crude tall oil, that is, an operationwherein the bulk of the tall oil including the fatty acids and resinacids present therein are removed overhead and the pitch present in thecrude tall oil is removed from the bottom or base of the apparatus. Inthis depitching operation, low retention time avoiding thermaldegradation is important. Additionally, the apparatus can be used in theevaporation of ammonium nitrate of low water content in order to removefurther amounts of water. Here again, low residence time is desirable.Finally, the apparatus can be used in removing water or other relativelylow boiling materials from nylon salt, styrene monomer, and the like.

What is claimed is:

1. Apparatus suitable for use in evaporations involving heat sensitiveliquids comprising a central shaft, means for rotating the shaft, aplurality of rotors fastened to the shaft one above the other, each ofsaid rotors being in the form of a substantially fiat platesubstantially perpendicular to the axis of the shaft and having aserrated outer edge thereby defining a plurality of portions along saidouter edge, each of said portions being disposed angularly with respectto the plane of the rotor, means for feeding liquid to the top rotor,collecting means for collecting liquid projected from a rotor andfeeding it to the rotory directly below it, said collecting meanscomprising the interior wall of an external indirect heat exchanger, andbafiiing means associated with the collecting means adapted to arrestthe rotary component of motion of the liquid after it has been projectedfrom the rotor,

-2. Apparatus according to claim 1 in which said collecting means arecollecting cones.

'3. Apparatus according to claim 2 in which the diameter of said rotorsis less than the smallest diameter of said collecting cones.

4. Apparatus according to claim 3 in which said collecting conescomprise the interior wall of an external heat exchanger.

5. A method of evaporating a liquid which comprises repeatedlycentrifugally projecting the liquid from a coaxial source of supply insuccessively lower planes against successively lower levels of a closedsurface generally transverse to the planes of projection, supplying heatby indirect heat-exchange to the liquid while it is in contact with theclosed surface, collecting the liquid and arresting the rotary motionthereof after it has been projected against the closed surface, andfeeding the liquid to the next lower level of centrifugal projection ofthe liquid.

References Cited in the file of this patent UNITED STATES PATENTS BowenNov. 20, 1928 Yarrnett Oct. 22, 1929 Yarmett Feb. 9, 1932 Valentine May3, 1932 Baird Feb. 11, 1936 'Nilsson Mar. 12, 1946 Adams Apr. 22, 1952Dunning et al. Jan. 29, 1957 Wistrioh et al. Aug. 27, 1957 M asek et a1June 24, 1958 Walley et al. Nov. 1 0, 1959 FOREIGN PATENTS Germany Feb.23, 1893 Great Britain Jan. 17, 1951 Italy Mar. 17, 1952

1. APPARATUS SUITABLE FOR USE IN EVAPORATIONS INVOLVING HEAT SENSITIVELIQUIDS COMPRISING A CENTRAL SHAFT, MEANS FOR ROTATING THE SHAFT, APLURALITY OF ROTORS FASTENED TO THE SHAFT ONE ABOVE THE OTHER, EACH OFSAID ROTORS BEING IN THE FORM OF A SUBSTANTIALLY FLAT PLATESUBSTANTIALLY PERPENDICULAR TO THE AXIS OF THE SHAFT AND HAVING ASERRATED OUTER EDGE THEREBY DEFINING A PLURALITY OF PORTIONS ALONG SAIDOUTER EDGE, EACH OF SAID PORTIONS BEING DISPOSED ANGULARLY WITH RESPECTTO THE PLANE OF THE ROTOR, MENS FOR FEEDING LIQUID TO THE TOP ROTOR,COLLECTING MEANS FOR COLLECTING LIQUID PROJECTED FROM A ROTOR ANDFEEDING TI TO THE ROTORY DIRECTLY BELOW IT, SAID COLLECTING MEANSCOMPRISING THE INTERIOR WALL OF AN EXTERNAL INDIRECT HEAT EXCHANGER, ANDBAFFLING MEANS ASSOCIATED WITH THE COLLECTING MEANS ADAPTED TO ARRESTTHE ROTARY COMPONENT OF MOTION OF THE LIQUID AFTER IT HAS BEEN PROJECTEDFROM THE ROTOR.